1. Technical Field
The present invention relates to a four-stroke engine, and more specifically to a lubricating technology for a four-stroke engine mounted in a portable working machine such as a concrete cutter or a backpack working machine such as a mist blower.
2. Related Art
Lubrication apparatuses as disclosed in, for example, Japanese Patent Application Laid-Open No. 2004-251231 and Japanese Patent Application Laid-Open No. 2011-069240 have been known, which are used in a four-stroke engine mounted in a working machine.
The lubrication apparatus disclosed in JP2004-251231 performs oil circulation as follows: sucking up the oil accumulated in the oil tank into the crank chamber to lubricate the crank chamber; directly supplying the oil to the cam gear and the rocker arm to lubricate them; and returning the oil to the oil tank. This type of lubrication apparatus directly supplies oil from the crank chamber to valve operating members such as cam gears, and therefore the performance of lubricating the valve operating members can be kept. On the other hand, since a large amount of oil needs to be circulated, problems may occur in a gas-liquid separator that separates oil from blowby gas. The oil for lubrication is reused while being prevented from deteriorating through the following process: first, liquefying oil mist and separating the liquefied oil from blowby gas; delivering the separated blowby gas to a combustion chamber; and collecting the liquefied oil into the lubrication apparatus. Here, if too much oil is circulated, oil together with blowby gas supplied from the gas-liquid separator to the combustion chamber. This causes oil to be consumed quickly. When this type of engine is turned around in different directions in use, it promotes oil consumption, and therefore is not suitable for use in a portable working machine. In addition, this type of engine has advantages when used in a stationary or in-vehicle working machine, but not suitable for use in a portable working machine.
Meanwhile, the lubrication apparatus disclosed in JP2011-069240 performs oil circulation as follows: sucking up the oil accumulated in the oil tank into the crank chamber to lubricate the crank chamber; turning the oil into oil mist; collecting the oil mist in the oil tank once to reduce the concentration of the oil mist; and supplying the oil mist with the reduced concentration to valve operating members to lubricate the valve operating members. This type of lubrication apparatus circulates oil mist with a reduced concentration, and therefore exhibits good performance of a gas-liquid separator that separates oil from blowby gas. Therefore, it is very useful to save oil consumption. On the other hand, the performance of lubricating the valve operating members is not good, and therefore it is not suitable for use in a large working machine that is required to output high power. A cam has been commonly used as a valve operating member. However, the cam is particularly prone to be worn among valve operating members, and therefore high lubrication performance is required. A large engine cannot easily release heat, and therefore, if the temperature rises, the viscosity of the oil adhering to the driving parts decreases. This may cause oil depletion. If the performance of lubricating the driving parts degrades, friction is created. This causes the driving parts to be damaged quickly. This type of engine has a small displacement of 40 cc or less, and is particularly advantageous when used in a small portable working machine that is turned around in different directions in use, such as hedge trimmer, a chain saw, an olive harvester and so forth.
This type of engine is advantageous when used in a small portable working machine, but is not suitable for a large working machine.
However, a four-stroke engine having a displacement of 50 cc or more is being used in a portable working machine and a backpack working machine. Although these types of working machines also are turned around indifferent directions in use, each of them has a weight greater than one having a displacement of 40 cc or less, and therefore is not very often shaken violently in use. However, if the type of the engine as disclosed in JP2004-251231 is used in a working machine, problems occur in the blowby gas separation, and therefore oil is consumed quickly. Meanwhile, if the type of the engine as disclosed in JP2011-069240, the performance of lubricating the valve operating members degrades, so that the valve operating members may be damaged quickly.